Biology Reference
In-Depth Information
A
No one knows why robins lay blue eggs, but the eggs of most thrush species are blue.
Thebluedoesn'tseemtocamouflagetheeggs,butitdoesn'tmakethemparticularlycon-
spicuous either. Pigments usually add some structural strength to tissues, and this partic-
ular blue pigment may give the eggshells just the right thickness and hardness to protect
the embryo while allowing it to finally break out.
Robins flock year-round except during the breeding season. The mother remains at or
near the nest site all the time when she has eggs or nestlings, but after the first brood of
young fledge, the father takes them at nighttime to a communal roost. When he leaves
them after 12 or 14 days to help raise the next brood, they remain together and with other
young robins.
Afterthepairshavefinishedraisingyoung,theyalsojointheseflocks.Duringfalland
winter, robins can sometimes be found in flocks numbering in the thousands, ten thou-
sands, or even hundred thousands, though most flocks seem to number a dozen or two.
These flocks break up in spring when males and females appear restless and territorial
squabbles erupt more frequently.
The differences between male and female robins are very subtle. Females are overall
paler than males, especially on the head, which is almost black on males. The white eye
crescents and throat striping are a bit more dramatic on males, too. But when robins from
other areas join together, especially in winter flocks, sometimes you'll notice that bright
femalesfromoneareaareasintenseasdullmalesfromanotherarea.It'seasiesttoseethe
differences between a pair that you're regularly watching, especially during the breeding
season. Also, if you hear a robin singing (not just making the
peek!
or
tut-tut-tut
calls),
it's a male for sure.
BOYS AND GIRLS TOGETHER
The male and female of some species of birds,
especially those that mate for life,
such as jays, crows, ravens, cranes, eagles, geese, and swans, are identical or differ only
in size. In other species, the male and female may bear dramatically different plumage,
suchascardinals,waxwings,finches,andmanyducks.Thisiscalledsexualdimorphism.
Females are often fairly cryptically colored, which helps camouflage them on the
eggs,thoughmaleRose-breastedGrosbeaks,despitetheirbrilliantcolorandpattern,also
incubate. Males are bright, which helps them attract a mate and aids in their territorial
defense.Forexample,femaleRed-wingedBlackbirdsandfemaleBaltimoreOriolestend
toprefermales withthemostbrilliant colors,andthese males tendtodefendthebestter-
ritories.
